The present invention relates to a pulse width detector for detecting the pulse width of a digital signal reproduced from a recording medium and a digital player using the detector.
FIG. 1 shows a motor drive system of a digital audio disc player. Numeral 1 denotes a disc, numeral 2 denotes a disc drive motor, numeral 3 denotes a pickup (sensor) for picking up a signal from the disc, and numeral 4 denotes a synchronizing signal detector for detecting a synchronizing signal from the signal reproduced from the disc. Since the synchronizing signal is recorded on the disc at a constant period, it is an indicator of rotational speed. The frequency of the synchronizing signal is converted to a voltage by a frequency-to-voltage converter 5, which voltage is then fed back to the motor 2 to form a servo loop.
The signal on the disc 1 comprises a series of pulses as shown in FIG. 2. The width of each pulse is an integral multiple of a basic period T. Numeral 6 denotes a specific pattern in the pulse sequence, which has a combination of high-level and low-level pulse widths of 11T. It is the synchronizing signal distinguished from other information signals. The pulse width of 11T is a maximum pulse width of all pulse widths.
By utilizing a synchronizing signal having the pulse width of 11T--11T as a velocity signal, a velocity control of the disc motor can be attained. However, the synchronizing signal is obtained only during a steady rotation state, and the pulse width of 11T cannot be obtained at the time of start of the motor because it is rotated at a low rotational speed. As a result, it is not possible to detect the rotational speed under these starting conditions. A method for effecting velocity control at the time of starting of the motor using the output signal of a frequency-to-voltage converter has been proposed.
An alternative method is disclosed in a copending U.S. application Ser. No. 311,048 filed Oct. 13, 1981 entitled "CLOCK RATE DETECTOR" and assigned to the present assignee, which issued as U.S. Pat. No. 4,423,498 on Dec. 27, 1983. In the alternative method, a pulse having a maximum width is detected from randomly detected pulses having various pulse widths. It corresponds to the pulse width 11T in the steady state. The velocity is predicted from the maximum pulse width. That is, an error in the pulse width of 11T is detected.
The signal detected from the disc is close to an analog signal at an early state of the rotation and it is converted to a digital signal by slicing it at a predetermined level. The reproduced signal is shown by 7 in FIG. 3. Numerals 8, 9 and 10 denote slicing levels for the digital conversion. When the slicing level is near a center of an amplitude (peak-to-peak) of the reproduced signal 7 as shown by the level 9, the duty factor of the digital-converted pulse waveform is approximately 50% so that the pulse widths of 11T are obtained. However, when the slicing level is shifted upward or downward as shown by the levels 8 and 10, the duty factor is not 50% and the pulse widths of 11T are not exactly obtained.
When the reproduced waveform is not vertically balanced as shown in FIG. 4, the exact pulse widths of 11T are also not obtained. In FIG. 4, the pulse widths of 11T are obtained from a waveform 15 but the exact pulse widths of 11T are not obtained from the reproduced waveforms 14 and 16 because of vertical unbalance.
When the start control for the disc is to be effected in accordance with the pulse width of the digital pulse, the motor cannot be controlled from the start of rotation to steady rotation unless the exact pulse width is obtained. Accordingly the detection of the exact pulse width is essential.
If an amplitude variation or a bias variation is included in a binary "1" and "0" input signal of the digital signal reproduced from the disc, data is not correctly discriminated when the level of the input signal varies with a constant level of threshold voltage. Accordingly, in a prior art binary signal data discriminator, a peak and a bottom of the input signal are detected so that the level at a mid-point thereof is compared to exactly discriminate the data.
FIG. 5 shows waveforms for explaining the operation of the prior art circuit. Numeral 21 denotes an input waveform, numeral 22 denotes the level at a mid-point of the peak and the bottom of the input waveform 21, and numeral 23 denotes a digital signal derived by voltage comparison of the input waveform 21 at the mid-point level 22. Under this condition, even if a D.C. bias variation or an amplitude variation at a sufficiently low frequency to the input waveform 21 is included, the digital signal 23 is not influenced. However, if a vertically asymmetric waveform distortion is included as shown by the input waveform 24 of FIG. 6, the mid-point level is changed to a level 25 and a digital signal 26 which is different from a desired digital signal 27 is produced. As a result, the data cannot be exactly discriminated. See, for example, Japanese Patent Laid-Open Nos. 55-150644 and 55-150645.